1. Field of the Invention
The present invention relates to a method and apparatus for multiplexing a video signal.
2. Related Background Art
The advancement of electronic technology is remarkable such technology is used in various fields. One of the advantages of using electronic technology is the ability of instantly transmitting signals such as various sounds, images or character data, which are essential in the present information-oriented world.
The signals which were transmitted in analog form in the past, are now being replaced by digital transmission because of advantages in the transmission quality. Audio signals are now commonly processed digitally. Advantages of the digital signals are that signal deterioration on a transmission line is low, high quality transmission is attained, cross-talk-free multiplexing is attained, mass transmission is attainable by using a light transmission technique such as an optical fiber, and signal processing which cannot be attained for an analog signal may be done by a computer. Audio signals may be digitized and transmitted with a data signal by a time division multiplex system. For example, in Japanese Laid-Open Patent Application 61-9044, a composite video signal including a digital sub-signal multiplexed by an audio signal and a data signal is PF-modulated for light transmission.
Recently, a trial has been made to digitize, not only an audio signal, but also a video signal for multiplex transmission. For example, Japanese Laid-Open Patent Application 2-141195 relates to a digital audio data format which facilitates multiplexing with a digital video signal. It includes measurement of data destroyed by the digital sub-signal which most likely occurs on a long distance transmission line. The multiplexed signal of the digital video signal and the digital sub-signal is usually a serial data stream having a format as shown in FIG. 1. In this format, a data stream 604, of the digital sub-signal, and a sync pattern 602 are added to a data stream 603, of the digital video signal to form one frame 601.
In order to separate the digital video signal and the digital sub-signal from the multiplexed signal, an apparatus as shown in FIG. 3 may be used. An operation of the apparatus is briefly explained below. When the sync pattern 602 is detected by a frame sync circuit 912, a control signal, synchronized with the sync pattern 602, is supplied to a decoder 911. In the decoder 911, the multiplexed signal is converted to a parallel signal, and an output timing of the parallel signal is timed by a control output from the frame sync circuit 912 so that it is separated to the digital video signal 904 and the digital sub-signal 902. Since the output of the decoder 911 is a parallel signal, it is supplied to a D/A converter 913 when an analog video signal output is required.
In Japanese Laid-Open Patent Application 61-148983, the digital sub-signal is temporarily stored in a memory. A special pattern for identification is added to the digital sub-signal read from the memory, and the digital sub-signal having the special pattern added to a portion corresponding to a sync signal period of the analog video signal is added to the data stream of the digital video signal to form one frame. FIG. 2 shows the frame In this method, the amount to be transmitted is reduced over than that of the format of FIG. 1.
When the multiplex transmission is performed by the format of FIG. 1, the amount to be transmitted increases as described in Japanese Laid-Open Patent Application 61-148983. Differential encoding of the video signal is done to prevent such an increase but, the differential encoding needs additional reproducing unit in a receiving station and the size of the apparatus increases.
Further, in the method of adding the digital sub-signal to the portion corresponding to the sync signal period of the analog video signal, to the data stream of the digital video signal as shown in FIG. 2, high access speed memories and circuitry for controlling the memories at a high speed are required both in the transmitting station and in the receiving station. In this case, the pattern design must take the high speed requirement and stable operation into consideration, and hence the design is hard to attain. Further, the digital sub-signal is reproduced with one frame delay.
Various methods have been proposed for video signal multiplexing transmission and a best method is being studied.